Since most plastics derived from petroleum are light in weight, tough and durable and can be molded easily and arbitrarily, they are mass-produced to support our lives in many ways. However, when these plastics are thrown away into the environment, they are not easily decomposed and are accumulated. When they are burnt, they release a large amount of carbon dioxide, thereby accelerating global warming.
In view of this situation, researches into resins obtained from non-oil raw materials or biodegradable plastics which are degraded by microorganisms are now actively under way. The biodegradable plastics have an aliphatic carboxylate unit and are easily degraded by microorganisms. However, they have low heat stability and a serious problem such as a reduction in their molecular weights or the deterioration of their colors in the molding step where they are exposed to a high temperature, such as melt spinning, injection molding or melt film formation.
Although polylactic acid, out of these, is a plastic which has excellent heat resistance and good balance between hue and mechanical strength, as compared with petrochemical-based polyesters typified by polyethylene terephthalate and polybutylene terephthalate, it has lower heat resistance, whereby it has a problem that it cannot be ironed when it is formed into a fabric.
To overcome this situation, various studies have been made on the improvement of the heat stability of polylactic acid. One of the studies is a stereocomplex polylactic acid. The stereocomplex polylactic acid is a polylactic acid containing a stereocomplex crystal and having a melting point 30 to 50° C. higher than that of an ordinary polylactic acid containing a homocrystal.
However, the stereocomplex crystal does not always appear, and a homocrystal often appears in a high molecular weight region. Even a polylactic acid composed of only a stereocomplex crystal may contain a homocrystal when it is crystallized after it is re-molten. To improve this phenomenon, studies are being made on a crystal nucleating agent for growing only a stereocomplex crystal.
For instance, Patent Document 1 teaches that a mixture obtained by mixing together chloroform/hexafluoro-2-propanol solutions of poly(L-lactic acid) and poly(D-lactic acid) having a weight average molecular weight (may be abbreviated as Mw hereinafter) of about 120,000 in the presence of an oxamide derivative is a polylactic acid composed of only a stereocomplex crystal as a result of DSC measurement.
Patent Document 2 teaches that when an aromatic urea-based compound is used, a polylactic acid composed of only a stereocomplex crystal is obtained.
However, when a stereocomplex polylactic acid is produced by these processes, as a large mount of a halogen-containing organic solvent is used, a recovery process is required and an environmental load becomes significant. When a stereocomplex polylactic acid is to be produced by melt kneading to eliminate these, as the oxamide derivative and the aromatic urea-based compound are nitrogen-containing compounds, a molecular weight reduction becomes a problem and it is difficult to obtain a stereocomplex polylactic acid having an Mw of not less than 150,000.
Further, Patent Document 3 teaches a process of producing a multi-block copolymer comprising poly(L-lactic acid) and poly(D-lactic acid) having an Mw of less than 100,000 and a relatively short chain length and discloses that the copolymer is a polylactic acid containing only a stereocomplex crystal. However, as re-precipitation must be carried out each time the number of blocks of the copolymer is increased, the copolymer is not suitable for industrial-scale production.
As described above, there is unknown a process of producing a polylactic acid having an Mw of more than 100,000 from which only a stereocomplex crystal is grown even by repeating melting and crystallization.    (Patent Document 1) JP-A 2005-255806    (Patent Document 2) JP-A 2005-187630    (Patent Document 3) JP-A 2002-356543